The present invention relates to drilling apparatus and more particularly to an overshot assembly for retracting and/or lowering drilling tools, including a core barrel inner tube assembly in a drill string.
Auld et al U.S. Pat. No. 2,347,726 discloses an overshot assembly having a jar head connected to the jaw assembly by jars. From the drawings it appears that one of the jars has an elongated slot into which the other jar extends. Bryson U.S. Pat. No. 1,593,430 discloses a drill jar piston that includes a piece of wire rope extending between the head and body portions. A metal sleeve surrounds the wire rope and abuts against the head and body portions to form a rigid spacer. It is further indicated that the drill jar piston is of a type shown in Bryson's earlier U.S. Pat. No. 1,200,465. Keller U.S. Pat. No. 4,061,389 discloses an overshot having a head portion connected by a jar to a knuckle joint which in turn is connected to a jaw assembly.
In using prior art overshot and core barrel inner tube assemblies, for example such as disclosed in U.S. Pat. No. 2,829,868 to Pickard et al, if the coupled assemblies are sufficiently elevated by the wire line to completely clear the drill string and the inner (lower) end of the inner tube assembly is then moved away from the drill string, as the coupled assemblies are lowered for laying the assemblies flat on the earth surface, at times conventional spearpoints such as disclosed in Pickard will pivot in the direction of the overshot dogs whereby the inner tube assembly becomes uncoupled and the outer end of the inner tube assembly falls. This can result in injury to the operator. Also, in using the above procedure with an overshot assembly such as disclosed in the patent to Pickard, at times, the jar staff bends or breaks whereby the jar staff has to be replaced. In an attempt to overcome such problems there was made a knuckle joint spearhead for core barrel inner tube assemblies, see U.S. Pat. No. 4,281,725 to Runk.
The knuckle joint core barrel inner tube assembly of Runk has been in use for many years. During that period of time, at times some problems have been encountered. If an adaptor sleeve is provided in surrounding relationship to the pulling dogs of an overshot assembly in order to adapt the overshot assembly for use in a larger inner diameter drill string than that with which the overshot assembly was originally intended to be used, in heavy drill mud, the rate of descent in a drill string is slower than without the sleeve. However, the sleeve is provided to help maintain the overshot assembly in a generally centered position in the drill string. Further, over a period of time, the spring in the knuckle joint device such as disclosed in Runk wears such that the point subassembly may tip in the drill string relative to the spear head base which makes it more difficult, if not impossible, for the overshot assembly to couple to the drilling tool that is to be retracted. Additionally, with the overshot and inner tube assemblies elevated out of the drill string, as the inner end of the inner tube assembly is manually moved during the step that the inner tube assembly is laid flat on the supporting surface, the transverse pivot axis of the knuckle joint is sufficiently angled relative to the supporting surface, the point subassembly and spear head base do not pivot relative to one another as intended. This can result in the jar rod being bent whereby it has to be replaced.
Further, in prior art overshot assemblies, the jar staff, which is provided to permit, in many situations, delivering hammering blows to facilitate coupling to an inner tube assembly and/or facilitate retracting the latches after the overshot assembly has coupled to the core barrel inner tube assembly, is the weak link and fails due to bending. Additionally, in the event that problems in the bore hole cause drill rods to twist off, it is easier to fish out an inner tube assembly that has a fixed spearhead (non-knuckle joint) as contrasted to one having a knuckle joint.
In order to make improvements in overshot apparatus to overcome problems such as the above, this invention has been made.